Method and apparatus for sensing and analyzing electrical activity of the human heart

ABSTRACT

A method of sensing and analyzing electrical activity of the human heart comprises the sensing of voltage signals generated by the heart between four electrodes located at key positions on the surface of a subject&#39;s body. Signal processing means combines and scales the voltage signals to produce xyz vectorcardiographic signals, electrocardiographic signals corresponding to the lead signals of a 12-lead electrocardiograph, or both.

FIELD OF THE INVENTION

This invention relates to electrophysiology, and more particularly toinstrumentation and methods for sensing and analyzing activity of thehuman heart.

BACKGROUND OF THE INVENTION

Over the last several decades, a variety of diagnostic procedures havebeen developed for sensing and analyzing activity of the human heart.These include electrocardiography, vectorcardiography andpolarcardiography, all of which depend upon related instrumentation usedto produce records derived from voltages produced by the heart on thesurface of the human body.

The records so produced are graphical in character and requireinterpretation and analysis to relate the resulting information to theheart condition of the patient or other subject. Historically, suchrecords have been produced directly as visible graphicalrecordings--wired connections extending from the subject to therecording device. With advances in computer technology, it as becomepossible to produce such records in the form of digitally storedinformation for later replication or retrieval and analysis. Likewise,with advances in communication technology, remote (wireless) sensing hasbecome possible.

(a) Electrocardiography

The production of a conventional 12-lead electrocardiogram (ECG)involves the placement of 10 lead electrodes (one of which is a groundor reference electrode) at selected points on the surface of a subject'sbody. Each electrode acts in combination with one or more otherelectrodes to detect voltages produced by depolarization andrepolarization of individual heart muscle cells. The detected voltagesare combined and processed to produce 12 tracings of time varyingvoltages. The tracings so produced are as follows:

    ______________________________________                                        Lead Voltage        Lead    Voltage                                           ______________________________________                                        I    vL - vR        V1      v1 - (vR + vL + vF)/3                             II   vF - vR        V2      v2 - (vR + vL + vF)/3                             III  vF - vL        V3      v3 - (vR + vL + vF)/3                             aVR  vR - (vL + vF)/2                                                                             V4      v4 - (vR + vL + vF)/3                             aVL  vL - (vR + vF)/2                                                                             V5      v5 - (vR + vL + vF)/3                             aVF  vF - (vL + vR)/2                                                                             V6      v6 - (vR + vL + vF)/3                             ______________________________________                                         where, in the standard, most widely used system for making short term         electrocardiographic recordings of supine subjects, the potentials            indicated above, and their associated electrode positions, are:               vL potential of an electrode on the left arm;                                 vR potential of an electrode on the right arm;                                vF potential of an electrode on the left leg;                                 v1 potential of an electrode on the front chest, right of sternum in the      4th rib interspace;                                                           v2 potential of an electrode on the front chest, left of sternum in the       4th rib interspace;                                                           v4 potential of an electrode at the left midclavicular line in the 5th ri     interspace;                                                                   v3 potential of an electrode midway between the v2 and v4 electrodes;         v6 potential of an electrode at the left midaxillary line in the 5th rib      interspace;                                                                   v5 potential of an electrode midway between the v4 and v5 electrodes;         vG (not indicated above) is a ground or reference potential with respect      to which potentials vL, vR, vF, and v1 through v6 are measured. Typically     though not necessarily, the ground or reference electrode is positioned o     the right leg.                                                           

Correct interpretation of an ECG requires a great deal of experiencesince it involves familiarity with a wide range of patterns in thetracings of the various leads. Any ECG which uses an unconventionalsystem of leads necessarily detracts from the body of experience thathas developed, in the interpretation of conventional ECGs, and maytherefore be considered generally undesirable. The tracings generatedwould be understandable only by a relative few who were familiar withthe unconventional system.

Nevertheless, other leads system have evolved from improvements ininstrumentation that have permitted extension of electrocardiography toambulatory, and even vigorously exercising subjects--and to recordingsmade over hours, or even days. For example, in stress testing theelectrodes are moved from the arms to the trunk, although the samenumber of electrodes (10) are used. The tracings I, II, III, aVR, aVLand aVF are altered by this modification.

Although a 12-lead ECG is considered to be a cost effective heart test,it is to be noted that the relatively large number of electrodesrequired play an important role in determining costs--not only in termsof the direct cost of the electrodes themselves, but also terms of thetime required to properly position and fix each electrode to a subject'sbody.

(a) Vectorcardiography

The pattern of potential differences on a body surface resulting fromelectrical activity of the heart can be mathematically approximated byreplacing the heart with a dipole equivalent cardiac generator. Themagnitude and orientation of this dipole are represented by the heartvector which is continually changing throughout the cycle of the heartbeat. The XYZ coordinates of the heart give rise to time varying xyzsignals, which may be written out as xyz tracings. Orthogonal leads togive these tracings were developed by Ernest Frank (see An Accurate,Clinically Practical System For Spatial Vectorcardiography, Circulation13: 737, May 1956). Frank experimentally determined the image surfacefor one individual, and from this proposed a system using sevenelectrodes on the body, plus a grounding electrode. The conventionalletter designations for such electrodes, and their respective positionswere:

E at the front midline;

M at the back midline;

I at the right mid-axillary line;

A at the left mid-axillary line;

C at a 45° angle between the front midline and the left mid-axillaryline;

F on the left leg;

H on the back of the neck.

The first five electrodes (E, M, I, A and C) were all located at thesame transverse level--approximately at the fourth of the fifth ribinterspace. A linear combining network of resistors attached to theseelectrodes gave suitably scaled x, y and z voltaage signals as outputs.

Unfortunately, xyz tracings are not as easy to interpret as 12 leadECGs. However, Frank intended his system for a different purpose:vectorcardiography.

Vectorcardiography abandons the horizontal time coordinate of the ECG infavour of plots or tracings of the orientation and magnitude of theheart vector on each of three planes: a frontal (xy) plane plotting anx-axis (right arm to left arm) against a y-axis (heat to foot); atransverse (xz) plane plotting the x-axis against a z-axis (front toback), and a sagittal plane plotting the y-axis against the z-axis.

Although it has long formed a basis for teaching electrocardiography,vectorcardiography has never become widely used. The technique wasdemanding and the system of electrode placement was different from thatrequired for the ECG. Extra work was required, and it would still benecessary to record a 12-lead ECG separately with a different placementof electrodes.

An alternative to the Frank lead sysftem that required only four activeelectrodes (R(right arm), A, F, E), and that used a resistor networkbased on Frank's image surface data was proposed in 1958 by G. E. Dower(the inventor herein) and J. A. Osborne (see A Clinical Comparison ofThree VCG Lead Systems Using Resistance-Combining Networks, Am Heart J55: 523 1958). However, the xyz signals produced were sometimesdifferent from those of Frank's lead system, and the RAFE system was notadopted.

(c) Polarcardiography

An alternative representation of the heart vector, known aspolarcardiography, has been exploited since the early 1960's (see G. E.Dower, Polarcardiography, Springfield, Ill, Thomas, 1971). It hascertain inherent advantages in defining abnormalities, and forms thebasis of a successful program for automated analysis. Based on xyzsignals, polarcardiography employs the Frank lead system. In order torender it competitive with the established 12-lead ECG, the lead vectorconcept has been employed to derive a resistor network than wouldlinearly transform the xyz signals into analogs of the 12-lead ECGsignals (see G. E. Dower, A Lead Synthesizer for the Frank Lead Systemto Simulate the Standard 12-Lead Electrocardiogram, J. Electrocardiol 1:101, 1968; G. E. Dower, H. B. Machado, J. A. Osborne, On Deriving theElectrocardiogram From Vectorcardiographic Leads, Clin Cardiol 3: 97,1980; and G. E. Dower, The ECGD: A Derivation of the ECG from VCG leads(editorial), J. Electrocardiol 17: 189, 1984). The ECG thus [d]erived iscommonly referred to as the ECGD. Because the ECGD can be acceptable toan interpreting physician, it is not necessary for the technician toapply the electrodes required for a conventional ECG. Further,associated computer facilities can make vectorcardiograms and otheruseful displays available from the xyz recordings. Nevertheless, thenumber of electrodes called for by the Frank lead system are required.In addition, the effort required by the technician recording the xyzsignals is about the same as for a conventional ECG.

The primary object of the present invention is to provide a method andapparatus for sensing and analyzing activity of the human heart, andwhich requires a reduced number of electrodes to produce accuratesimulations of conventional 12-lead electrocardiograms andvectorcardiograms.

SUMMARY OF THE INVENTION

It has been found that accurate simulations of 12-leadelectrocardiograms can be derived by measuring and processing voltagessensed using 4 electrodes strategically placed on the surface of asubject's body. Although a fifth or grounding electrode may also berequired, this will depend on the equipment used and may be avoided withsuitable equipment.

As will become apparent, it is also possible to derive vectorcardiogramsusing the same 4 electrode placements. Indeed, in one implementation ofthe present invention, 12-lead electrocardiographic signals are derivedfrom xyz vectorcardiography signals, the latter of which are derivedfrom voltages sensed using the basic 4 electrodes. With thisimplementation, the derivation of vectorcardiographic signals may beseen as an intermediate result or step. However, as will be seenhereinafter, while the intermediate derivation of xyzvectorcardiographic signals is possible, it is not essential to thederivation of 12-lead electrocardiographic signals.

The 4 electrode positions that are fundamental in the use of the presentinvention consist of electrode position E of the Frank lead system,electrode position A of the Frank lead system, an electrode position Sover the upper end of the sternum (manubrium sterni), and electrodeposition I of the Frank lead system. (Such E, A, S and I electrodes arefrom time-to-time collectively referred to herein as the "EASI"electrodes.)

It has been found that 12-lead electrocardiographic sgnals and xyzvectorcardiographic signals can be derived by measuring and, withsuitable signal processing means, combining and scaling the voltagespresent between first, second and third selected pairs of the EASIelectrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the EASI electrode arrangement with signal processingfor deriving xyz vectorcardiographic signals.

FIG. 2 illustrates the EASI electrode arrangement with signal processingfor deriving 12-lead electrocardiographic signals, xyzvectorcardiographic signals being derived at an intermediate stage.

FIG. 3 illustrates the EASI electrode arrangement with signal processingderiving 12-lead electrocardiographic signals.

FIG. 4 illustrates in more detail a signal processing means for derivingvectorcardiographic and electrocardiographic signals from EASIelectrodes.

FIG. 5(a) representationally depicts a Frank network forming part of thebedside recorder shown in FIG. 4, the input of such network beingconfigured to receive signals from EASI electrodes in accordance withthe present invention.

FIG. 5(b) representationally depicts the Frank network as shown in FIG.5(a), but with a normal (prior art) input configuration to receivesignals from conventionally positioned E, M, I, A, C, F and H electrodesas taught by Frank.

FIG. 6 is a prior art circuit diagram of a surrogate patient devicewhich forms part of the signal processing means in FIG. 4.

FIG. 7 is a circuit diagram of linear transformation circuits that formcorrecting network 402.

DETAILED DESCRIPTION

FIGS. 1, 2 3 and 4, each depicting a generalized signal processing meansor signal processor 100, 200, 300 or 400, as the case may be, serve toillustrate the placement of EASI electrodes on the surface of asubject's body at positions E, A, S and I as hereinbefore described. (Inthis drawing, the letters E, A, S and I are used not only to identifypositions on the human body but also electrodes at such positions.)

The electrodes themselves are common, widely available electrodes usedin the process of takin ECGs, VCGs and the like. Their respectivepositions E, A, S and I serve to avoid the limbs and other undesirablesites such as the C, M and H positons (in the region of the left nipple,the back, and the head or lower neck) as used by Frank. Further, theygive good signals with relatively little noise.

FIG. 1 illustrates a generalized signal processor 10 receiving EASIelectrode signals (via wire connections 1, 2, 3 and 4) as an input, andproducing xyz vectorcardiographic signals as an output.

FIG. 2 illustrates a signal processor 200 having a first stage 201 and asecond stage 202. In this case, 9 output signals are derived to serve asinputs for producing a 12-lead ECG. At the intermediate stage of theoutput of stage 201 and the input of stage 202, xyz vectorcardiographicare produced. FIG. 2 recognizes that 12-lead electrocardiographicsignals may be derived from xyz vectorcardiographic signals. This is ofcourse the basis for the ECGD described above and is not a newobservation per se. However, the derivation of 12-leadelectrocardiographic signals from derived xyz vectorcardiographicsignals produced in the manner indicated through stage 201 in FIG. 2 isconsidered new. In principle, it will be noted that there is nofundamental difference between signal processor 100 of FIG. 1 and firststage 201 of signal processor 200 in FIG. 2.

As will become apparent hereinafter, the signal processing means thatare used in the implementation of the present invention comprise linearvoltage combining and scaling networks. This is true of signal processor100 in FIG. 1, and is also true of stages 201 and 202 of signalprocessor 200 shown in FIG. 2. Given this premise, it will be readilyapparent to those skilled in the art that stages 201 and 202 in FIG. 2may in fact be condensed into a single stage. FIG. 3 highlights the factthat this may be done. Here, signal processor 300 derives 12-leadelectrocardiographic signals directly from EASI electrode signal inputs,and there is no necessary production of derived intermediate xyzvectorcardiographic signals as in the case of signal processor 200.Implicitly, signal processor 300 could include such a facility, but itis not essential.

FIG. 4, like FIG. 2, shows means for deriving both vectorcardiographicand electrocardiographic signals from EASI electrodes, signal processor400 of FIG. 4 performing essentially the same function as signalprocessor 200 of FIG. 2. Additionally, FIG. 4 illustrates connection toan electrocardiograph 500. A fifth or grounding electrode G is alsoshown in FIG. 4, as is a ground line or path generally designated 5. Theplacement of grounding electrode G is not critical; it may be placedanywhere convenient--though typically on a subject's chest as indicatedin FIG. 4. In any given case, the necessity for a grounding electrodeand a ground line will depend on the equipment utilized. FIG. 4illustrates such elements because they were used in the particular casenow to be described in more detail.

Signal processor 400 comprise a bedside recorder 401, a correctingnetwork 402, and a surrogate patient device 403. The combination ofrecorder 401 and correcting network 402 may be thought of a first stage201 in FIG. 2. Surrogate patient device 403 may be thought of as secondstage 202 in FIG. 2. Recorder 401, correcting network 402, and surrogatepatient device 403 all act as liner voltage combining and scalingnetworks.

A preliminary point of note is that bedside recorder 401 and surrogatepatient device 403 as stand alone elements are esentially well knowndevices. The recorder used is a TOTEMITE™ Bedside Recorder, whichembodies a processing network to derive conventional xyzvectorcardiographic signals in accordance with Frank (see above) andwhich is commonly used to record such signals on magnetic tape. Whenused in the manner indicated in FIG. 4, signals normally directed tomagnetic tape (and which are normally xyz signals) are tapped to provideinput signals (x'y'z') for correcting network 402. This isrepresentationally shown in FIG. 5(a) which depicts a Frank network 405as an included part of bedside recorder 401, but wired at the input toreceive 4 EASI electrode signals. Conventional use of the Frank networkis illustrated in FIG. 5(b) which shows each of the network's seveninput terminals A, C, E, I, M, H and F present for the purpose ofreceiving inputs from each of seven corresponding A, C, E, I, M, H and Felectrodes (not shown) located at corresponding A, C, E, I, M, H and Fpositions on a subject (also not shown).

Surrogate patient device 403 is a linear processing network whoseoutputs are scaled to match those that electrocardiographic 500 would"see" from electrodes attached to a subject for a conventional ECG. Thedesign of such networks is known, the result being an ECGD. FIG. 6illustrates prior art circuitry used to provide the action of asurrogate patient monitor. Since such design is known it will not bedescribed here in any detail. However, it may be noted that the circuitarrangement shown in FIG. 6 is essentially disclosed in XYZ DataInterpreted by a 12-Lead Computer Program Using the DerivedElectrocardiogram, J. Electrocardiol 12: 249, 1979 (by G. E. Dower andH. B. Machado). In that disclosure, signals again taken from a TOTEMITEBedside Recorder (but using the Frank network forming part of therecorder "normally" to obtain xyz vectorcardiographic signals) wereamplified by a factor of 1000 and applied to the XYZ terminal inputs ofthe surrogate patient device. In the environment of the presentinvention as shown in FIG. 4, the bedside recorder still receives xyzvectorcardiographic signal inputs; the difference now is that they arenot received directly as an amplified output of the bedside recorder,but instead as the output of correcting network 402.

Referring again to FIG. 4 and FIG. 5(a), it can be appreciated that EASIelectrode E is connected not only to input E of the Frank network 405,but also input C of the Frank network. Likewise, EASI electrode A isconnected not only to input A of the Frank network, but also input M.EASI electrode I is connected not only to input I of the Frank network,but also input F. EASI electrode S is connected to input H of the Franknetwork. By reason of the departure from the 7 electrodes normallyproviding input to the Frank network, and by reason of the differinginput configuration to the Frank network, it follows naturally that onewould not expect the usual xyz vectorcardiographic signals at the outputof network.

More particularly, the conventional input/output signal relationshipwith a Frank network is:

    v.sub.x =0.610v.sub.A +0.171v.sub.C -0.781v.sub.I          (1)

    v.sub.y =0.655v.sub.F +0.354v.sub.M -1.000v.sub.H          (2)

    v.sub.z =0.133v.sub.A +0.736v.sub.M -0.264v.sub.I -0.374v.sub.E -0.231v.sub.C                                             (3)

v_(x), v_(y) and v_(z) appear as potential differences at the threepaired outputs of Frank network 500 in FIG. 5(b). v_(A), v_(C), v_(I),etc. are measured with respect to an arbitrary reference of potentialchosen by Frank. Any one of the seven electrodes of the Frank leadsystem could have been selected as the reference potential.

When the EASI electrode system is used as the input to the Frank networkas shown in FIG. 5(a), the I electrode serves as the reference. (Itshould be noted that the selection of the I position as the reference isnot essential. Any one of the four EASI electrode positions may serve asthe reference). With the I position as the reference, there are threeinput potential differences v_(AI), v_(EI) and v_(SI) as sensed betweenelectrode pairs A-I, E-I and S-I, respectively. The input/output signalrelationship of the Frank network with the input configuration modifiedas shown in FIG. 5(a) becomes:

    v.sub.x' =0.610v.sub.AI +0.171v.sub.EI

    v.sub.y' =0.354v.sub.AI -1.000v.sub.SI

    v.sub.z' =0.869v.sub.AI -0.605v.sub.EI.

The signals produced, herein are referred to as x'y'z' signals(illustrated as voltage signals v_(x'), v_(y') and v_(z') in FIG. 5(a)),contain sufficient information to derive xyz vectorcardiographicsignals. The accessability of this information is enhanced by goodsignal strengths and low noise ratios associated with the EASI electrodepositions on a subject's body.

It is of course key to the present invention that such information ispresent and extractable from signals sensed by the EASI electrodes. Alsokey is the ascertainment of transformation coefficients which enable oneto take EASI electrode signals and produce xyz vectorcardiographicand/or electrocardiographic signals. In the case of signal processor 400shown in FIG. 4, signals x'y'z' essentially act as a "given" and thetransformation or "correction" to xyz signals is performed by correctingnetwork 402, the circuit design of which is shown in FIG. 7.

Each of the three circuits shown in FIG. 7, are basic operationalamplifier circuits designed to perform linear input/outputtransformations. As an aside, it should be noted that the input act asvoltage sources and are assumed in FIG. 7 to be ideal sources havingzero impedance. In practice, this will not be the case. Typically, itwill be 100Ω and this value has been assumed as part of the inputresistor values shown in FIG. 7. Thus, 100Ω must be subtracted from theresistor values shown in FIG. 7 to obtain the actual resistance valuesof discrete input resistors.

From the resistor values shown in FIG. 7, it may be readily determinedthat the three circuits shown will perform to solve the following threeequations:

    v.sub.x =1.118v.sub.x' +0.109v.sub.z'                      (1)

    v.sub.y =-0.051v.sub.x' +0.933v.sub.y' -0.087v.sub.z'      (2)

    v.sub.z =-1.108v.sub.x' +0.772v.sub.z'                     (3)

where x, y and z are xyz vectorcardiographic signals. However,implementation of the present invention does not start with theforegoing equations or with the equation solving circuits shown in FIG.1; it starts with a determination of what the coefficients in theforegoing equations should be, and then proceeds with the routine designof circuitry which operates to solve the equations.

A statistical method of determining such coefficients has been devised,and has been found to work remarkably well. Both x'y'z' and xyz signalsare sampled at corresponding times from a number of subjects. With theaid of a computer, the samples may be compared using standardleast-square methods to extract correlating coefficients. The resultingequations express x, y and z in terms of x', y' and z'.

The coefficients shown in equations (1), (2) and (3) above result fromthe foregoing procedure as first applied to 27 subjects, and then testedby taking new ECGDs with the arrangement illustrated in FIG. 4. TheseECGDs compared favourably with ECGDs obtained directly with the Franklead system.

On a further 16 subjects, ECGDs obtained utilizing EASI electrodes havebeen taken and compared favourably with ECGs obtained with conventionalelectrocardiographic equipment with the limb electrodes moved to thetrunk positions used for stress testing. Taking the ECG obtained withthe electrodes on the limbs as the standard, the ECGDs from the EASIelectrode positions resembled the standard ECG either as closely as ormore closely than the ECG obtained with all electrodes on the trunk.This result is of particular interest because it makes the ECGDacceptable for stress testing.

Because the application of electrodes for stress testing requirescareful preparation of the skin, and because the electrodes are usedonly once, the requiement of only five EASI electrodes (including aground electrode), connected with the ten normally needed, results in asaving of time and money. Furthermore, by reason of their locationfavouring large signals and relatively small amounts of movementartifact or signal noise, the quality of the tracings obtained tends tobe improved. These benefits can be obtained without modification toexisting equipment. However, it will be appreciated that with suitableequipment a variety of displays (including vectorcardiography andpolarcardiography) becomes obtainable from xyz signals without modifyingthe recording technique--the record in all cases being taken from EASIelectrode positions.

It will also be apparent to those skilled in the art that the discreteelements represented by Frank network 405 in FIG. 5(a) and the activeresistive circuits shown in FIG. 7 can be condensed to provide a moredirect derivation of xyz signals from the EASI electrode signals whichare the input to network 403 This would forego the immediate advantageof utilizing a commercially available recorder, but recognizes that theoverall network can readily be condensed and embodied in a singlepatient's cable. The same is true if one extends the process to includecircuitry of surrogate patient device 403.

The foregoing detailed description of various elements of the presentinvention is not intended to be limiting as to the spirit and scope ofthe invention as defined in the following claims.

I claim:
 1. Instrumentation for measuring and processing voltagesproduced by a human heart as sensed between selected points on thesurface of a subject's body, said instrumentation comprising:(a) a firstelectrode attachable to the anterior midline of the subject's body at alevel selected from the group consisting of:(i) the fourth ribinterspace; (ii) the fifth rib interspace; (b) second and thirdelectrodes attachable to the subject's body on opposed sides of theanterior midline at the same level as said first electrode; (c) a fourthelectrode attachable over the subject's manubrium sterni; and, (d)signal processing means operatively connected to said electrodes forreceiving first, second and third electrical signals present betweensaid first, second and third electrodes as produced by said heart atsaid points of attachment; and for combining and scaling said signals toproduce xyz vectorcardiographic signals in response thereto. 2.Instrumentation as defined in claim 1, including a ground electrodeoperatively connected to said signal processing means and attachable toa preselected location on the subject's body.
 3. Instrumentation asdefined in claim 2, wherein said preselected location is on thesubject's chest.
 4. Instrumentation as defined in claim 1, said signalprocessing means including means for receiving said vectorcardiographicsignals and for producing electrocardiographic output signalscorresponding to lead signals I, II, III, aVR, aVL, aVF, V1, V2, V3, V4,V5 and V6 of a 12-lead electrocardiogram in response thereto. 5.Instrumentation for measuring and processing voltages produced by ahuman heart as sensed between selected points on the surface of asubject's body, said instrumentation comprising:(a) first electrodeattachable to the anterior midline of the subject's body at a levelselected from the group consisting of:(i) the fourth rib interspace;(ii) the fifth rib interspace; (b) second and third electrodesattachable to the subject's body on opposed sides of the anteriormidline at the same level as said first electrode; (c) a fourthelectrode attachable over the subject's manubrium sterni; and, (d)signal processing means operatively connected to said electrodes forreceiving first, second and third electrical signals present betweenfirst, second and third pairs of said electrodes as produced by saidheart at said points of attachment; and for producingelectrocardiographic output signals corresponding to lead signals I, II,III, aVR, aVL, aVF, V1, V2, V3, V4, V5 and V6 of a 12-leadelectrocardiogram in response thereto.
 6. Instrumentation as defined inclaim 5, including a ground electrode operatively connected to saidsignal processing means and attachable to a preselected location on thesubject's body.
 7. A method of sensing and analyzing activity of a humanheart comprising the steps of:(a) sensing voltage signals generated bythe human heart between four electrodes positioned on the surface of asubject's body at, respectively,(i) the anterior midline of thesubject's body at a level selected from the group consisting of:(A) thefourth rib interspace; (B) the fifth rib interspace; (ii) opposed sidesof the anterior midline of the subject's body at the same level as saidfirst electrode; (iii) over the manubrium sterni of the subject's body;and, (b) combining and scaling such voltage signals to produceelectrocardiograph output signals corresponding to lead signals I, II,III, aVR, aVL, aVF, V1, V2, V3, V4, V5 and V6 of a 12-leadelectrocardiogram.
 8. A method as defined in claim 7, further comprisingthe step of:(a) combining and scaling said voltage signals to firstproduce xyz vectorcardiographic signals; and (b) combining and scalingsaid xyz vectorcardiographic signals to produce saidelectrocardiographic output signals.
 9. A method of sensing andanalyzing activity of the human heart comprising the steps of:(a)sensing voltage signals generated by the human heart between fourelectrodes positioned on the surface of a subject's body at,respectively,(i) the anterior midline of the subject's body at a levelselected from the group consisting of:(A) the fourth rib interspace; (B)the fifth rib interspace; (ii) opposed sides of the anterior midline ofthe subject's body at the same level as said first electrode; (iii) overthe manubrium sterni of the subject's body; and, (b) combining andscaling such voltage signals to produce xyz vectorcardiographic outputsignals.